The present invention relates to a system and method for measuring voltage and high frequency resistance of fuel cell stacks. More particularly, the present invention relates to system and method for measuring individual cell voltages and resistance of a fuel cell stack in which cells are stacked in series.
A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen or air is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:
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xe2x80x83xc2xdO2+2H++2exe2x88x92xe2x86x92H2O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.
Various parameters have to be monitored to ensure the proper operation of a fuel cell stack and evaluate the performance thereof. These parameters include the voltage across each fuel cell in the fuel cell stack, hereinafter referred to as cell voltage, and the internal resistance of each fuel cell.
The literature indicates that complex impedance measurements on fuel cells can only be performed using expensive bench-top laboratory equipment, consisting of may sub-systems interfaced with on another. For example, T. E. Springer, T. A. Zawodzinski, M. S. Wilson and S. Gottesfield, xe2x80x9cCharacterization of polymer electrolyte fuel cells using AC impedance spectroscopyxe2x80x9d, Journal of the electrochemical Society of America, 143(2), p. 587-599, 1996; J. R. Selman and Y. P. Lin, xe2x80x9cApplication of AC impedance in fuel cell research and developmentxe2x80x9d, Electrochemica Acta, 38(14), p. 2063-2073, 1993; B. Elsener and H. Bolmi, xe2x80x9cComputer-assisted DC and AC techniques in electrochemical investigations of the active-passive transitionxe2x80x9d, Corrosion Science, 23(4), p. 341-352, 1983. Such known equipment is manually controlled, with no automation in place. No single known approach allows the use of a portable, integrated measurement system. In addition, no measurement equipment is integrated into these systems which permits modification of fuel cell operating parameters.
Furthermore, the patent literature shows that the measurement of complex impedance is primarily known for use on batteries. In addition, these patents only teach techniques for measuring a single quantity, namely xe2x80x9cimpedancexe2x80x9d (U.S. Pat. Nos. 4,697,134 and 5,773,978) or xe2x80x9cresistancexe2x80x9d (U.S. Pat. Nos. 3,753,094, 3,676,770 and 5,047,722). The previously mentioned patent relating to measuring impedance of an electrochemical cell (U.S. Pat. No. 6,002,238), not necessarily a fuel cell, used an entirely different, yet complicated approach. Furthermore, this approach could not be directly applied to fuel cells due to high currents associated with the latter.
Thus, there are still issues that need to be addressed, such as portability, fuel cell applicability, measurement variety, resolution, automation and cost. These issues have been addressed, to some extent, in the assignee""s co-pending U.S. patent application Ser. No. 09/672,040 that provides a self-contained, portable apparatus/system for measuring fuel cell impedance and a method of the same. The system comprises a CPU, frequency synthesizer, a fuel cell, a load bank and measurement and acquisition circuitry. The CPU receives input parameters from a software program and sends the parameters to a signal generation device which produces an AC waveform with a DC offset that is used to remotely program a load bank. The load bank draws current from the fuel cell. The voltage across the fuel cell and the current through the fuel cell are measured by voltage and current sensing circuitry, then digitized and averaged by an oscilloscope or A/D converter. The recorded data is sent to the CPU where the AC phase lead or lag is calculated. Numerous outputs can then be displayed by the invention, including real impedance, imaginary impedance, phase difference, leading component, lagging component, current magnitude, voltage magnitude and applied AC voltage.
However, the invention of that earlier application has limited application in measurement of fuel cell stacks consisting of a large number of fuel cells, where voltage measurement may be difficult using conventional measuring devices. A scheme for measuring the internal resistance of individual fuel cells within a fuel cell stack in a real-time manner is not detailed in the previous patent application.
In order to measure cell voltages, differential voltage measurement is required at the two terminals (i.e. anode and cathode) of each fuel cell. However, since fuel cells are connected in series, and typically in large number, the voltages at some terminals will be too high for any currently available semiconductor measuring device to directly measure. For example, for a fuel cell stack consisting of 100 cells with each cell voltage at 0.95 V, the actual voltages on the negative terminal (cathode) of the top cell will be 94.05 V (i.e. 0.95*100xe2x88x920.95). As such, the voltage exceeds the maximum allowable input voltage of most current differential amplifiers commonly used for measuring voltage.
The assignee""s co-pending U.S. patent application Ser. No. 09/865,562 provides a solution for this problem. This patent application provides a system for monitoring cell voltages of individual fuel cells in a fuel cell stack; the contents of both U.S. patent application Ser. Nos. 09/865,562 and 09/672,040 are hereby incorporated by reference. The system comprises a plurality of differential amplifiers, a multiplexer, an analog to digital converter, a controller and a computer. Each of the differential amplifiers reads the voltages at two terminals of each fuel cell. The analog to digital converter reads the output of the differential amplifiers via the multiplexer, which provides access to one of these differential amplifiers at any given time. The digital output of the analog to digital converter is then provided to the computer for analysis. The computer controls the operation of the analog to digital converter and the multiplexer. However, the voltage monitoring system in this patent application only measures the DC voltage across individual fuel cells. In contrast, in the aforementioned U.S. patent application Ser. No. 09/672,040, the measurement of fuel cell impedance involves applying both AC and DC voltages across a complete fuel cell stack, whether this is a single fuel cell or a stack of many fuel cells.
Therefore, there is still need for a system that is suitable for measuring internal resistance of individual fuel cells within a fuel cell stack, especially a stack consisting of a large number of fuel cells.
According to one aspect of the present invention, there is provided a system or apparatus for measuring fuel cell voltage and impedance. The apparatus comprises:
a voltage measuring means including a plurality of inputs for connection across a plurality of measuring points between cells of the electrochemical device, to generate voltage signals indicative of the measured voltages, the voltage measuring means providing a plurality of primary channels for the measured voltages, there being one channel for the voltage across adjacent measuring points; and wherein the voltage measuring means includes a channel splitter for separating out components of the measured voltages across adjacent measuring points from the primary channels, the channel splitter having first channels for providing DC components of the measured voltages and second channels for providing AC components of the measured voltages;
a load, connectable in series with the electrochemical device, and adapted to draw a DC current with a superimposed AC perturbation current; and,
a controller connected to and controlling the voltage measuring means and the load, for controlling the characteristics of the load and for receiving the voltage signals from the voltage measuring means, wherein the controller is adapted to control the load to provide desired DC and AC load current characteristics.
Preferably, the voltage measuring means includes a plurality of instrumentation amplifiers connected to the inputs of the voltage measuring means and having outputs providing the plurality of the primary channels and an analog multiplexer connected to at least the first channels from the channel splitter, wherein a multiplex control line is connected between the controller and the analog multiplexer for controlling the analog multiplexer to switch sequentially between at least the first channels.
It will be understood that the voltage signals could simply be the actual voltage across each cell.
According to another aspect of the present invention, there is provided a method of monitoring the voltage and impedance characteristics of cells of a multi-cell electrochemical device, the method comprising:
(i) providing a load connected in series with the electrochemical device;
(ii) controlling the load to provide desired current characteristic, comprising a desired combination of a DC current and a superimposed AC current perturbation, according to a series of set load conditions;
(iii) measuring the voltage across a plurality of measuring points between cells of the electrochemical device by providing a plurality of primary channels, there being one channel for the voltage across adjacent measuring points, and separating out components of the measured voltages across adjacent measuring points from the primary channels by providing at least one first channel for obtaining DC components of the measured voltages and at least one second channel for obtaining AC components of the measured voltages; and
(iv) recording at least some of the measured voltages.
For both aspects of the invention, the voltages measured need not be across each individual cell. It is possible that voltages could be measured across just some of the cells, and/or some individual voltages could be measured across a group of cells.